An image forming apparatus such as a copier, a printer, a facsimile, and a multi-function printer having a plurality of functions of such apparatuses using an electro-photographic or electrostatic recording system is configured to visualize (develop) an image by applying developer to an electrostatic latent image formed on an image carrier such as a photosensitive drum. Conventionally, some developing apparatuses used in such development are known to use two-component developer (referred to simply as ‘developer’ hereinafter) composed of toner, i.e., non-magnetic particles, and carrier, i.e., magnetic particles.
In such a developing apparatus, the developer is carried on a surface of a developing sleeve and is conveyed as the developing sleeve rotates. While being regulated in amount (layer thickness) by a regulating blade, i.e., a developer regulating member, disposed in close proximity with the developing sleeve, the developer is conveyed to a developing area facing a photosensitive drum. Then, an electrostatic latent image formed on the photosensitive drum is developed by the toner within the developer.
Here, as the developing apparatus, there is well known a developing apparatus having a so-called function separated type configuration including a supply chamber (first chamber) supplying the developer to the developing sleeve and a recovery chamber (second chamber) recovering the developer from the developing sleeve. There is also known a developing apparatus having a so-called carrier refreshing configuration of replenishing new developer and of discharging extra developer from a discharge port because the carrier deteriorates and drops its charging performance during its use. A configuration of executing the following control is known as the function separated type developing apparatus having the carrier refreshing configuration.
For instance, Japanese Patent Application Laid-open No. 2011-53451 proposes an arrangement (mode) of reducing a ratio of rotational speed Vsc of a conveying screw circularly conveying the developer in the supply and recovery chambers and rotational speed Vsl of the developing sleeve in order to remove a toner layer generated at an upstream location of the regulating blade. That is, if the image forming operations are carried out continuously, toner deposits upstream of the regulating blade, thus forming the toner layer. Because the toner layer narrows down a gap between the developing sleeve and the regulating blade, it causes a conveyance failure hampering the supply of the developer to the developing sleeve. Then, Japanese Patent Application Laid-open No. 2011-53451 proposes to remove the toner layer by temporarily interrupting the image forming operation and by executing the above-mentioned control (mode) when the image forming operations are continuously carried out.
Still further, Japanese Patent Application Laid-open No. 2010-152098 proposes a control (mode) of driving the conveying screw while retarding the rotational speed of the developing sleeve to be slower than that during the image forming operation in order to restrain the developer from overflowing out of the developing apparatus even if the developer is unevenly distributed in one chamber of the supply and recover chambers. For instance, in a case of a configuration in which the recovery chamber is disposed under the supply chamber in the function separated type developing apparatus, the developer is liable to be unevenly distributed in the recovery chamber. If the developer is tried to be recovered to the recovery chamber from the developing sleeve in such state, there is a possibility that the developer overflows out of the recovery chamber. Then, Japanese Patent Application Laid-open No. 2010-152098 conveys the developer from the recovery chamber to the supply chamber and discharges the developer forcibly from a discharge port of the supply chamber by executing the above-mentioned control (mode) in forming no image.
However, there is a case when it is unable to fully restrain the toner layer from being generated upstream of the regulating blade even if the developing sleeve and conveying screws are driven in forming no image in driving conditions different from those in forming an image as disclosed in Japanese Patent Application Laid-open No. 2011-53451. Then, there is a need for a developing apparatus capable of fully restraining the toner layer from being generated upstream of the regulating blade with a simple structure.
Still further, when the controls (modes) described in the above-mentioned Japanese Patent Application Laid-open Nos. 2011-53451 and 2010-152098 are executed, the distribution of the developer in the supply and recovery chambers are differentiated from a normal distribution. That is, in the case when the control (mode) described in Japanese Patent Application Laid-open No. 2011-53451 is executed, the distribution of the developer becomes different and the developer is stored more in the recovery chamber than the supply chamber, differing from the distribution in forming an image. Still further, as compared to a stationary state, i.e., the distribution of the developer in forming an image, spots where a developer face level has risen are partly generated. If an image is started to be formed in this condition, the developer at the spots in which the developer face level has risen is very likely to be conveyed to the discharge port without collapsing, thus possibly ending up being excessively discharged.
Still further, if the control (mode) described in Japanese Patent Application Laid-open No. 2010-152098 is executed, a height of the developer face level in the supply chamber increases, so that there is also a possibility of discharging the developer excessively from the discharge port of the supply chamber if an image is formed as it is.
For the foregoing reasons, there is a need for a developing apparatus capable of restraining such troubles otherwise caused by such control of disturbing the distribution of the developer face level within the supply and recovery chambers in forming no image, even when such controls (mode) are executed.